Extraction of metal ions from an aqueous solution

Abstract: Use of a porous, preferably dimensionally stable, hydrophobic polymer for the extraction of metal ions from an aqueous solution, with the polymer being in the form of a regenerative granulate or powder having an average particle diameter of 0,1 to 10 mm and pores of 0,1 to 50 µm, which is obtained by the dissolving of a polymer, with heating, at a temperature above the upper critical decomposition temperature Tc in a complexing agent containing hydrophobic liquid, followed by lowering of the temperature, resulting in a porous polymer filled with the hydrophobic liquid, and the mechanical diminution of the solidified mass. Favourable results have been attained using polypropylene as the hydrophobic polymer, bis(2,4,4-trimethylpentyl)thiophosphinic acid as the complexing agent, and with a mixture of dibenzyl toluene and/or polypropylene glycol ether being part of the hydrophobic liquid.

Description: The invention relates to the use of a porous, preferably dimensionally stable, hydrophobic polymer for the extraction of metal ions from an aqueous solution, the polymer having pores in which there is immobilised an organic complexing agent having hydrophobic properties which optionally is dissolved in a hydrophobic liquid. Such a use, with a porous polymer being employed, was earlier described in US-A-4 437 994. The porous polymer is in the form of hollow fibres, the walls of which consist of a microporous membrane having its pores filled with an organic liquid comprising a complexing agent, and the ions travel through said walls to be liberated in an aqueous product solution circulated over the outside of the fibres. A major drawback to the use of said known materials is that their preparation is laborious and hence expensive. Prior to extrusion the melted polymer used for making melt spun fibres has been mixed with finely divided material, which is removed after forming of the fibre in the spinneret by dissolving such in an appropriate solvent in a template leaching process to impart porosity to the wall of the finished fibres. Said process is complicated and expensive and does not lead to fibres having an anisotropic nature, which is a prerequisite for high capacity, non-leaking fibres. Therefore, preference is given to a solution spinning process using a tube-in-orifice spinneret system to obtain fibres possessing an anisotropic structure, with, in effect, skins about the outside of the fibre and bounding the lumen within the fibre which have pores of relatively small size, and with the wall of the fibre between these inner and outer skins having distributed therein pores of considerably greater size. In the examples exclusively use is made of polysulphone microporous fibres, the preparation of which calls for the use of water compatible solvents, such as dimethyl formamide (DMF), dimethyl sulphoxide, tetrahydrofuran, and acetone, which may prove harmful to the environment to a greater or lesser extent. After their preparation, the microporous walls of the hollow fibres are impregnated with organic liquid by circulating the organic liquid through the lumens of the fibres for several minutes, during which time the liquid is absorbed by capillarity. Said filling operation is cumbersome and therefore expensive. A further drawback of using the known materials is the risk of fouling. In WO 91/05859 use is made of a porous material with an organic complexing agent immobilised therein for the extraction of metal ions. The porous matrix is composed of polysulphone or cellulose acetate, which are comparatively hydrophilic. Porosity is attained by dissolving the polymer in an organic solvent, adding the complexing agent thereto, and injecting the thus obtained solution into a water bath. Further, a certain amount of activated carbon is added to immobilise the complexing agent in the porous polymer. Nor is the low rate of exchange mentioned in several of the examples encouraging where commercial scale use of this material is concerned. GB-A-1 407 257 discloses a process for the preparation of ion-exchange materials in which organic ion-exchange reagents are incorporated into a vinyl monomer suspension polymerisation charge which is then polymerised to entrap the organic ion-exchange reagents in the bulk of an organic polymeric support. The process is, however, not susceptible of universal application to all organic ion-exchange reagents; moreover, the materials are found to have rather slow extraction kinetics and also suffer from the gradual diffusion of the organic ion-exchange reagent from the material. GB-A-2 172 293 discloses the use of solid, immobilised extractants, produced by absorbing (a) a liquid extractant which is not homopolymerisable onto (b) a porous, solid, particulate, substrate; adsorbing (c) a monomer onto the substrate; and polymerising the adsorbed monomer (c) thereby entrapping the liquid extractant (a) on substrate (b). Said extractants have been found to have still insufficient capacity and extraction kinetics. The invention now provides for the use of a porous polymer with a hydrophobic organic complexing agent contained therein which, a significant increase in the capacity per volume unit notwithstanding, does not give rise to stability problems, not even when utilised on an industrial scale. Its commercial scale preparation can be effected easily, inexpensively, and in an environment-friendly manner. Metals can be bonded with it and regenerated over a very short period of time. The invention consists in that for the porous polymer of the known type mentioned in the opening paragraph use is made of a regenerative granulate or powder of a porous polymer having an average particle diameter of 0,1 to 10 mm and pores of 0,1 to 50 µm, which is obtained by the dissolving of a polymer, with heating, at a temperature above the upper critical demixing temperature Tc in the hydrophobic complexing liquid, followed by lowering of the temperature, resulting in a porous polymer filled with the hydrophobic liquid, and the mechanical diminution of the solidified mass. The average pore diameter can be determined with advantage using mercury porosimetry in accordance with ASTM D 4284-83. It must be regarded as extremely surprising that the use of a porous material having both a hydrophobic surface area and an average pore diameter in the indicated range provides a system which can be regenerated and is so effective as to now render commercial scale use possible. It was found that, in general, optimum results can be obtained when using a material having an average pore diameter in the range of 0,2 to 15 µm. While the hydrophobic complexing agent may be incorporated into the pores of the hydrophobic polymer as such, preference is given to material where the complexing agent is incorporated into a hydrophobic liquid. According to the invention, the organic complexing agent has to be readily soluble in the hydrophobic liquid, which, of course, will be virtually insoluble in the aqueous solution to be extracted and so immobilised in the porous matrix as to be incapable of flowing out. It has been found that optimum immobilisation is ensured when use is made of material having an average pore diameter of 0,2 to 15 µm. According to the invention, preference is given to an absorption material in which both the complexing agent and any hydrophobic liquid present are non-volatile and have high thermal, hydrolytic, oxidative, and chemical stability. The glycerol esters of one or more, preferably unsaturated fatty acids may be indicated as examples of a hydrophobic liquid suitable to be used according to the invention. Under certain conditions regeneration with an acid may cause the aforementioned oils to hydrolyse. To prevent this problem, the invention provides a porous material incorporating as hydrophobic liquid a non-hydrolysable high boiling solvent, which has preferably a boiling point of at least 200°C. Good results have been obtained with the use of a porous polymer filled with an aromatic solvent, such as dibenzyl toluene, which is commercialised by Hüls under the trade designation Marlotherm-S, tetratoluene, which is an experimental product ex Hüls, oligomers of styrene, and co-oligomers of styrene and α-olefins. Good results have also been obtained with the use of a porous polymer filled with a high boiling aliphatic solvent, such as paraffin oil. Mixtures of aromatic and aliphatic solvents are also suitable. Within this framework favourable results have likewise been obtained via incorporation into the hydrophobic liquid of polypropylene glycol ether having a molecular weight in the range of 600 to 1500, optimum results being obtained using a polypropylene glycol ether having a molecular weight of about 1000. The porous polymer is obtained by a process disclosed in US-A-4 247 498. Examples of polymers deemed more or less suitable for practising the present invention are: low density polyethylene, high density polyethylene, polypropylene, polystyrene, acrylonitrile-butadiene-styrene terpolymers, styrene-acrylonitrile copolymers, styrene-butadiene copolymers, poly(4-methyl-pentene-1), and polybutene. Optimum results were obtained by using polymers based on polyolefins, with preference being given to the use of a polypropylene based porous material. Generally, the porous polymer is employed in the form of a granulate or a powder having an average particle diameter of 0,1 to 10 mm and may be used in a packed bed, a fluidised bed, or a tank with stirring. It has been found that with the material according to the invention the extraction of metal ions is already possible at a concentration of less than 1 wt.% of complexing agent, calculated on the combined amount of complexing agent and hydrophobic liquid. In actual practice, preference is given to a material where the amount of complexing agent incorporated into the hydrophobic liquid is at least 5 wt.%. Optimum results were attained using an amount of complexing agent in the hydrophobic liquid in the range of 10 to 50 wt.%. However, higher concentrations of complexing agent of up to 100 wt.% can also give favourable results. The organic complexing agents suitable for use in the materials according to the present invention belong to several classes of compounds. So far, favourable results have been obtained using organic phosphorus compounds, certain oxim compounds, and certain organic carboxylic acids. Organic phosphorus compounds are particularly suitable for removing copper, nickel, cobalt, cadmium, uranium, the transurania, iron and/or zinc from aqueous waste streams, and said oxim compounds were found to be especially suitable for removing copper. Exemplifying an organic carboxylic acid pre-eminently suitable for complexing zinc is a highly branched C9-C11 alkyl carboxylic acid commercialised by Shell under the trade designation "Versatic acid." The organic phosphorus compounds are advantageously selected from the group of organic phosphates, phosphoric acids, phosphonic acids, phosphinic acids, thiophosphates, thiophosphoric acids, dithiophosphoric acids, thiophosphonic acids, dithiophosphonic acids, thiophosphinic acids, and dithiophosphinic acids. Preference is given in this case to the use of one or more compounds from the group of di (2-ethylhexyl) phosphoric acid, bis(2,4,4-trimethylpentyl) phosphinic acid, bis(2,4,4-trimethylpentyl) thiophosphinic acid, and bis(2,4,4-trimethylpentyl) dithiophosphinic acid. According to the invention, the preferred oxim compounds generally belong to the group of hydroxyoxim compounds. Examples of this type of complexing agent include 2-hydroxy-5-alkyl benzaldehyde oxime, 2-hydroxyalkyl benzophenone oxime, 2,6-diformyl-4-alkyl phenol oxime, or 5,8-diethyl-7-hydroxy-dodecan-6-one oxime. Generally, the procedure used to prepare the extracting material according to the invention is described in US-A-4 247 498. However, the preferred preparative process is a variation on one described in GB-A-2 115 425, which proceeds as follows: first, 5-90 wt.% of a polymer is dissolved, with heating, at a temperature beyond the critical demixing temperature Tc, in 10-95 wt.% of one or, where necessary, more liquid and miscible compounds A and/or B and/or C, the selected A/B/C mixing ratio being such as will give phase separation on cooling, resulting in a polymer-rich and a polymer-poor phase. On further lowering of the temperature said phase separation structure will be fixed prior to the completion of phase separation due to the polymer vitrifying or crystallising, resulting in a porous polymer material filled with one or more of compounds A and/or B and/or C and pre-eminently suited to be used according to the invention, either as such or after a reduction if so desired. Usually, compound C is added when compounds A and/or B display such a high affinity for the polymer as will result in there not being any liquid/liquid phase separation on cooling prior to vitrification or crystallisation. Up to now, favourable results have been attained using polypropylene as matrix polymer and bis(2,4,4-trimethylpentyl)thiophosphinic acid as complexing agent, while the hydrophobic liquid is a mixture of dibenzyl toluene and polypropylene glycol ether having a molecular weight in the range of 600 to 1500. Needless to say, the materials according to the invention are suitable not just for removing heavy metals, such as Fe, Cu, Cd and/or Zn, from aqueous waste streams; another option is to employ them in the extraction of valuable metals, such as Au, Ag, and Pt, or radioactive materials, such as uranium, from usually highly dilute aqueous solutions. The invention will be further illustrated with reference to the following examples, which, of course, are not to be construed as limiting in any manner the scope thereof. Example I Use was made of a porous polymer obtained by heating a mixture of 30 parts by weight of polypropylene, 19,3 parts by weight of bis(2,4,4-trimethylpentyl)thiophosphinic acid (Cyanex 302, ex Cyanamid), 32,9 parts by weight of soybean oil, and 17,8 parts by weight of castor oil to 250°C until a homogeneous mass was obtained, followed by cooling to form a porous polypropylene filled with a mixture of complexing agent, soybean oil, and castor oil. After grinding, a porous powder was obtained which satisfied the following specification: In each of the following experiments a column (length 93 mm, diameter 8,3 mm) in the form of a packed bed was filled with approximately 2 g of absorption material having the aforementioned specification. In all cases this column was loaded with Zn-containing waste water of the following composition: Na 1260 ppm Ca 15 ppm Fe 1 ppm Mg 9 ppm Zn 18 ppm Four experiments were performed in which the pH of the water to be purified was varied by adding a particular amount of 1 N NaOH. The rate of flow in all cases was 1 ml/minute. A Zn-concentration in the effluent of > 0,1 ppm was taken as the breakthrough point. The material loaded in experiment no. 3 was regenerated with 21 ml of 30% H2SO4 at a rate of flow of the liquid of 0,1 ml/minute. After regeneration it was found that only 0,06 mg of Zn remained in the column. No elements apart from Zn and Fe were absorbed in significant quantities. Example II In a manner analogous to that disclosed in Example I, porous polymers filled with extraction material were prepared by heating a mixture of 30 parts by weight of polypropylene, 30 parts by weight of Cyanex 302, 30 parts by weight of dibenzyl toluene (Marlotherm S, ex Hüls), and 10 parts by weight of polypropylene glycol having a molecular weight of 1000 to a temperature of 250°C until a homogeneous mass was obtained, followed by cooling to form a porous polypropylene filled with a mixture of complexing agent, dibenzyl toluene, and polypropylene glycol. After grinding, a porous powder was obtained with an average pore diameter of 5 µm. A column (length 90 mm, diameter 8,4 mm) in the form of a packed bed was filled with 2,2 g of the above absorption material. The column was loaded with an aqueous solution of zinc sulphate (20 mg Zn/l) at a flow rate of 1 ml/minute. A Zn-concentration in the effluent of > 0,1 ppm was taken as the breakthrough point. The result as a function of the pH is given in Table II. Example III In a manner analogous to that disclosed in Example II, porous polymers filled with extraction material were prepared by heating a mixture of 30 parts by weight of polypropylene, 50 parts by weight of Cyanex 302, and 20 parts by weight of polypropylene glycol having a molecular weight of 1000 to a temperature of 250°C until a homogeneous mass was obtained, followed by cooling to form a porous polypropylene filled with a mixture of complexing agent and polypropylene glycol. After grinding, a porous powder was obtained with an average pore diameter of 10 µm. The same column as in Example II was filled with 2,2 g of the above absorption material. The column was loaded with Zn-containing waste water of the following composition: mg/l Zn 20 Fe 0,7 Ca 11 Na 1325 Al 10 Cl 110 Two experiments were performed with the material of this Example (experiments 11 and 12) and two with the material of Example II (experiments 13 and 14). The pH of the water to be purified was in the range of 3,52 to 3,57. The rate of flow was about 1 ml/minute or 4 ml/minute. A Zn-concentration in the effluent of > 0,1 ppm was taken as the breakthrough point. Example IV This experiment serves to show that the packed column disclosed in Example II could be loaded and regenerated several times without any substantial loss of activity. The concentration of the Zn-containing water was 280 mg Zn/l, the pH 3,5, and the flow 1 ml/minute during 3 hours. After a breakthrough, the column was rinsed with demi-water, followed by regeneration with 25% H2SO4 in countercurrent at a flow rate of 0,1 ml/minute. The successive amounts of Zn (in mg Zn/g Cyanex) absorbed by the column were as follows: 63,5, 75,3, 65,6, 65,0, 60,8, 68,9, 70,4, 71,0, 68,7, and 67,4 mg Zn/g Cyanex. The regeneration was for > 99% complete after elution with 12 ml H2SO4. From the results mentioned above, it appears that there is no loss of activity.

Claims: Use of a porous, preferably dimensionally stable, hydrophobic polymer for the extraction of metal ions from an aqueous solution, the polymer having pores in which there is immobilised an organic complexing agent having hydrophobic properties which optionally is dissolved in a hydrophobic liquid, characterised in that use is made of a regenerative granulate or powder of a porous polymer having an average particle diameter of 0,1 to 10 mm and pores of 0,1 to 50 µm, which is obtained by the dissolving of a polymer, with heating, at a temperature above the upper critical demixing temperature T c in the hydrophobic complexing liquid, followed by lowering of the temperature, resulting in a porous polymer filled with the hydrophobic liquid, and the mechanical diminution of the solidified mass. Use according to claim 1, characterised in that the average diameter of the pores is 0,2 to 15 µm. Use according to claim 1, characterised in that the hydrophobic liquid contains a non-hydrolysable, high boiling solvent. Use according to claim 3, characterised in that the high boiling solvent has a boiling point of at least 200 °C. Use according to claim 3, characterised in that the high boiling solvent is an aromatic solvent, such as dibenzyl toluene. Use according to claim 3, characterised in that the high boiling solvent is an aliphatic solvent, such as paraffin oil. Use according to claim 1, characterised in that the hydrophobic liquid contains an adjuvant, such as polypropylene glycol ether having a molecular weight in the range of 600 to 1500. Use according to claim 1, characterised in that the amount of complexing agent in the hydrophobic liquid is at least 5 wt.%, calculated on the combined amount of complexing agent and hydrophobic liquid. Use according to claim 8, characterised in that the amount of complexing agent in the hydrophobic liquid is in the range of 10 to 50 wt.%. Use according to claim 1, characterised in that the complexing agent is an organic phosphorus compound. Use according to claim 10, characterised in that the organic phosphorus compound is selected from the group of organic phosphates, phosphoric acids, phosphonic acids, phosphinic acids, thiophosphates, thiophosphoric acids, dithiophosphoric acids, thiophosphonic acids, dithiophosphonic acids, thiophosphinic acids, and dithiophosphinic acids. Use according to claim 10, characterised in that the organic phosphorus compound is selected from the group of di(2-ethylhexyl) phosphoric acid, bis(2,4,4-trimethylpentyl) phosphinic acid, bis(2,4,4-trimethylpentyl) thiophosphinic acid, and bis(2,4,4-trimethylpentyl) dithiophosphinic acid. Use according to claim 1, characterised in that the complexing agent is a hydroxyoxime. Use according to claim 13, characterised in that the hydroxyoxim is 2-hydroxy-5-alkyl benzaldehyde oxime, 2-hydroxyalkyl benzophenone oxime, 2,6-diformyl-4-alkyl phenol oxime, or 5,8-diethyl-7-hydroxy-dodecan-6-one oxime. Use according to claim 1, characterised in that the porous polymer is obtained by first dissolving, with heating, 5-90 wt.% of polypropylene at a temperature beyond the critical demixing temperature T c , in 10-95 wt.% of a mixture of bis(2,4,4-trimethylpentyl) thiophosphinic acid, dibenzyl toluene and polypropylene glycol ether having a molecular weight in the range of 600 to 1500, the mixing ratio being so chosen that phase separation will occur on cooling, resulting in a polypropylene-rich and a polypropylene-poor phase, optionally followed by mechanical reduction of the mass solidified by further cooling.

Inventors: BOS JOHANNES, MEER ABELE BROER VAN DER, MIDDELHOEK ERIK LEONARD, SCHOMAKER ELWIN, BOS, JOHANNES, MIDDELHOEK, ERIK LEONARD, SCHOMAKER, ELWIN, VAN DER MEER, ABELE BROER
Applicants: AKZO NOBEL NV, AKZO NOBEL N.V.
Application year: 1994
Main classification: B01J45
IPC codes: C08L71, C22B3, C08L101, B01J45